2. Definition: Y varies directly as x means that y = kx where k is the constant of variation. Another way of writing this is k = In other words: * As X increases in value, Y increases or * As X decreases in value, Y decreases.

3. Examples of Direct Variation: Note: X increases, 6 , 7 , 8 And Y increases. 12, 14, 16 What is the constant of variation of the table above? Since y = kx we can say Therefore: 12/6=k or k = 2 14/7=k or k = 2 16/8=k or k =2 Note k stays constant. y = 2x is the equation!

4. Note: X decreases, 30, 15, 9 And Y decreases. 10, 5, 3 What is the constant of variation of the table above? Since y = kx we can say Therefore: 10/30=k or k = 1/3 5/15=k or k = 1/3 3/9=k or k =1/3 Note k stays constant. y = 1/3x is the equation! Examples of Direct Variation:

5. Note: X decreases, 4, 16, 40 And Y decreases. 1,4,10 What is the constant of variation of the table above? Since y = kx we can say Therefore: 1/4=k or k = ¼ 4/16=k or k = ¼ 10/40=k or k = ¼ Note k stays constant. y = ¼ x is the equation! Examples of Direct Variation:

7. Is this a direct variation? If yes, give the constant of variation (k) and the equation. Yes! k = 6/4 or 3/2 Equation? y = 3/2 x

8. Yes! k = 25/10 or 5/2 k = 10/4 or 5/2 Equation? y = 5/2 x Is this a direct variation? If yes, give the constant of variation (k) and the equation.

9. No! The k values are different! Is this a direct variation? If yes, give the constant of variation (k) and the equation.

11. Using Direct Variation to find unknowns (y = kx) Given that y varies directly with x, and y = 28 when x=7, Find x when y = 52. HOW??? 2 step process 1. Find the constant variation k = y/x or k = 28/7 = 4 k=4 2. Use y = kx. Find the unknown (x). 52= 4x or 52/4 = x x= 13 Therefore: X =13 when Y=52

12. Given that y varies directly with x, and y = 3 when x=9, Find y when x = 40.5. HOW??? 2 step process 1. Find the constant variation. k = y/x or k = 3/9 = 1/3 K = 1/3 2. Use y = kx. Find the unknown (x). y= (1/3)40.5 y= 13.5 Therefore: X =40.5 when Y=13.5 Using Direct Variation to find unknowns (y = kx)

13. Using Direct Variation to solve word problems Problem: A car uses 8 gallons of gasoline to travel 290 miles. How much gasoline will the car use to travel 400 miles? Step One : Find points in table Step Two: Find the constant variation and equation: k = y/x or k = 290/8 or 36.25 y = 36.25 x Step Three: Use the equation to find the unknown. 400 =36.25x 400 = 36.25x 36.25 36.25 or x = 11.03

14. Using Direct Variation to solve word problems Problem: Julio wages vary directly as the number of hours that he works. If his wages for 5 hours are $29.75, how much will they be for 30 hours Step One: Find points in table. Step Two: Find the constant variation. k = y/x or k = 29.75/5 = 5.95 Step Three: Use the equation to find the unknown. y=kx y=5.95(30) or Y=178.50

15. Direct Variation and its graph With direct variation the equation is y = kx Note: k = the constant and the graph will always go through…

16. the ORIGIN!!!!!

17. Tell if the following graph is a Direct Variation or not. No Yes No No

18. No Yes Yes No Tell if the following graph is a Direct Variation or not. teachers.henrico.k12.va.us/math/HCPSAlgebra1/Documents/3-6/ directvariation .ppt